Active extensional tectonics along the southwest margin of the Colorado Plateau is the eastern boundary of Basin and Range extension. Large normal faults of Late Cenozoic age are reasonably well documented. The amount of offset on these faults and their structural style may change around the southwest corner of the Colorado Plateau. We hope to quantify and document the changes in structural style and fault offset. From regional data, it appears that the total offset should decrease from the west-facing margin of the Plateau in northwest Arizona to the south-facing margin in central Arizona.

Total fault offset will be calculated along three transects of approximately 60 km each, spanning from the western margin of the Colorado Plateau at the Grand Wash fault, inboard through the Hurricane and Toroweap fault systems. Structural cross-sections spanning the same transects will be used to characterize the style of the deformation and where it is occurring along these transects. Two additional transects across the southern margin of the Colorado Plateau that include the Big Chino fault and the Aubrey Hills fault, will also be constructed. A detailed comparison of these five transects will enable us to evaluate the hypothesis of decreasing faulting to the south, and if active faulting is stepping over toward the east to accommodate extension. In the course of this regional structural analysis, we will also be able to evaluate the state of knowledge regarding extension along the margin of the Colorado Plateau, and identify areas for future study.